The present invention relates to a hydraulic power steering apparatus for assisting in vehicle steering through a hydraulic cylinder.
For example, Japanese Laid-Open Patent Publication No. 2006-123580 discloses a hydraulic power steering apparatus for assisting in steering of a vehicle through a hydraulic cylinder. This type of power steering apparatus includes a flow control valve for controlling supply/drainage of hydraulic fluid for the hydraulic cylinder. The supply/drainage of the hydraulic fluid is controlled in correspondence with the level of steering torque, which is transmitted from a steering shaft to a rack shaft through a rack and pinion mechanism.
In this flow control valve, the mode of the supply/drainage of the hydraulic fluid for the hydraulic cylinder is determined in correspondence with the amount of torsion of a torsion bar. The torsion amount of the torsion bar corresponds to the relative difference between the phase of a shaft fixed to the steering shaft and the phase of a rotary valve connected to the shaft through the torsion bar.
The above-described conventional hydraulic power steering apparatus assists in steering operation by the driver appropriately. However, assist force, or, in other words, the supply/drainage mode of the hydraulic fluid for the hydraulic cylinder is determined based on the torsion torque of the steering shaft solely. This makes it impossible to change the assist force depending on various types of demands based on the operating state of a vehicle. As a result, there is a demand for a hydraulic power steering apparatus capable of changing the assist force produced by a hydraulic cylinder to meet various types of demands.